Decoding coated abrasives: A guide to selecting the ideal sandpaper
November 24, 2020 | 11:32 am CST
When it comes to navigating the world of coated abrasives, there are many variables to consider, and it can be complicated to identify the best solution to meet your needs. With all the options out there, and even just those available through Uneeda, it wouldn’t surprise us if you felt overwhelmed or discouraged in the search for the perfect abrasives. That’s why we’re here to help you understand the ins and outs of these products, to help you get what you need so you can get the best results in your finish, while minimizing material and labor costs and the time spent sanding.
In this, part 1 of a two-part series, we’ll cover an overview of the technical aspects of abrasives, to give you a primer of what the world of abrasives has to offer. In part two, we’ll cover a framework for how to apply the technical aspects to choose products that will work best for your particular needs. Part two also includes a matrix of a sampling of popular applications and recommendations for them.
Let’s jump in.
Part 1: Understanding the Products Themselves
Before getting into the specifics of how to choose the best abrasive for you, it’s important to understand the technical aspects of the abrasives, such as: the types of grains, how they function and what they are used for; the available backings; the formats of products; the special coatings and fillers; the grain coating and a basic understanding of grit. We have articles about many of these topics that go much more in depth than we’ll cover here – so check those out for further clarification and discussion.
Aluminum Oxide, Silicon Carbide, Ceramic, Zirconia, Blend of Grains
In general, these days all the grains you’ll find have been synthesized in a lab setting, and have thus been designed to be utilized for specific purposes. Our article “Abrasive Grains: Everything You Need to Know to Choose the Right One” covers this subject in much more depth, and our infographic offers an easy visual cheat sheet.
The most important things to know on the topic for our purposes here are the idea of “friability” – or the nature of the grain to break and make new sharp edges – and the basic overview of the applications in which we use these grains in general. When it comes to friability, not all grains are created equal. Some grains will become dull – which may not be what you want – for instance in woodworking, when a grain becomes dull, that can create an uneven or blotchy finish.
In general, for woodworking, we recommend aluminum oxide for most lower grit to medium grit applications, and silicon carbide or aluminum oxide for some finishing or higher grit applications.
For metalworking, automotive and other similar applications, the story is a bit more complicated – so, depending on your application, you might choose ceramic, zirconia, aluminum oxide or silicon carbide. We’ll go into this more later, but the ideal choice will depend on how aggressive of sanding you’re doing, the level of hardness of the metal and the particular application, such as removing rust/deburring vs prepping for finish or polishing. Both zirconia and ceramic grains require a combination of quite a hard surface/higher amount of pressure to initiate the friability of the grains, therefore it’s generally not recommended to use them on wood, where it would cause the grain to become dull, rather than fracture.
Paper, Cloth, Film, Foam
Coated abrasives – aka sandpaper products – can be made on different sorts of substrates, of which paper is only one. Each offers various advantages and disadvantages. Our article about abrasive backings, “Paper, cloth, film or foam? Choose the right backing to get the job done.“, goes into much more detail. Here is a brief summary:
Paper – generally offers one of the best finishes, but can be easily torn. Depending on the thickness of the paper, it can be more or less flexible and more or less durable. Paper is often the least expensive. Paper is good for hand and orbital sanding and can also be utilized on belt sanders.
Cloth – cloth products are often quite durable and can be utilized for heavy duty machine sanding. Cloth products come in various weights and some are very sturdy, while others are very flexible, to match contours and profiles. Because the cloth used in these products contains a weave, this can cause an uneven scratch that could cause issues in the finish. Cloth products are typically used in applications where durability and stock removal are the highest priorities, as well as in some cases where flexibility and durability are needed. Cloth generally comes in belts and discs. Cloth products are also typically rated for wet applications and can withstand grease; they are also washable.
Film – The film used for coated abrasives is generally seen to be the optimal combination of durability and flexibility for sanding discs. It provides a very flat surface for the grains, while also being resistant to tearing and able to bend to match profiles. These qualities mean it is good for both leveling/shaping and finishing applications, where orbital and hand sanding will be done. Film products (paper as well) can be combined with interface pads on orbital sanders, to aid in contour sanding, reduce swirls and reach hard to access places.
Foam – While coated abrasives on foam sponges can be used on white wood, they are generally used more frequently for finishing applications, such as sanding between coats of finish. Two types of foam are generally used – softer, more flexible and harder, less durable. Sponges are available in two types from Uneeda – those with hook and loop backing for use on orbital sanders, and those without – made simply for hand sanding applications.
Open, Semi Open, Closed
In this case, coatings refers to the percentage of space on the backing which is covered with grain. Our article, “Sandpaper Coatings Demystified”, goes into much more detail about this subject and why it is important, and how it impacts the sanding life of the product.
Open coat has the most open space, least grain coverage. Generally speaking, this allows for a cooler sanding experience and slower or less loading. This means, it would be better to use open or semi-open in applications prone to loading, such as softer woods and metals.
Closed coat has the most grain coverage. Generally a closed coat will produce a more even scratch and therefore a better finish. It’s also good for heavier stock removal, since there are physically more grains present to do the cutting.
Fillers and Chemical Coatings:
Stearate, Antistatic, Various Resins
Stearate is an external coating on some coated abrasives. It acts as a lubricant to reduce friction while sanding. The purpose of reducing friction is to reduce heat, which causes all sorts of issues, from loading to uneven finish. Learn more about loading and some issues with heat in this article. Stearate is sometimes described as if you’d put soap on the sandpaper, allowing the material you’re sanding to slide out from the grains, rather than load into the sandpaper as you sand.. Another benefit to reducing friction is reducing the opportunity for static to occur.
In addition to stearate, some products also include special antistatic fillers/coatings to specifically reduce static. Static electricity can be a big problem when sanding, as it can cause dust extraction to run amok. This is particularly bad in wide belt sanding, where static can cause dust to stick to the belt and within machine components – causing all sorts of other issues. Read more about this in our article about the top problems in wide belt sanding here. Suffice it to say that if the application you’ll be doing will be prone to generating static electricity due to friction, choosing a product with Antistatic properties will be beneficial.
Wide and Narrow Belts, Discs (PSA, Hook and Loop, with holes/without holes), Sheets, Sponges (hand sanding and for sanders), other special shapes.
In general, coated abrasives are available in various formats based on the machine style or whether hand sanding will be done. Choosing the format will likely be the most straightforward part of the process of identifying the right product for you, unless you are starting from the beginning in implementing a sanding strategy.
Generally speaking, at Uneeda, we offer our products in a variety of formats included: wide belts, narrow belts, rolls, discs, sheets (both for orbital sanders and for hand sanding), and sponges. Other formats are available in certain circumstances as well, such as brushes and more.
When it comes to discs, several standard sizes are available, 5 and 6” discs, and 3” discs are most common for random orbital sanders and are available with and without vented holes for machines with a dust extraction system.
Coarse, Medium and Fine, and beyond
Coated abrasives come in a variety of different grits – starting as low as P8 and going to P1200 and up. Lower number grits are coarser grains and will do more stock removal, whereas higher grit numbers are finer grains and will have a much smaller cut rate. In general, Uneeda recommends not skipping more than one grit, to achieve the ideal finish. The reason for this is that doing so may will lead to an uneven scratch patter, in which the only the peaks are removed, leaving deep grooves. To learn more about grit and determining a proper sequence, check out our article: Grit Sequence: Let (gr)it do the hard work for you. Part 2 of this article will discuss grit a bit further, but essentially, this will be important to understand, in order to know which grits will be required based on your application and what sequence you’ll follow.
Before making a product selection, it’s key to understand at least the basics of coated abrasives. This article just scratches the surface (no pun intended!) of these products, so be sure to review our other linked articles for more background on each subject, from grains and grit to coatings, fillers, and backings.
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